Generator for producing a width-modulated square voltage

ABSTRACT

Apparatus producing a width modulated square wave for use in a motor control circuit controlling the speed of an alternatingcurrent motor. A low frequency three-phase generator with stationary components develops three wave outputs integrated and summed with a high frequency wave signal, in a summation circuit, produced by a frequency generator. The sign of the outputs of the summation circuit is ascertained by zero level detectors and employed for controlling the motor speed. A direct-current control voltage is applied to the apparatus to control the amplitude of the high and low frequency waves.

United States Patent Inventors Arne Jensen Als; Torn Katrup Petersen,Nordborg, both of Denmark Appl. No. 755,538 Filed Aug. 27, 1968 PatentedSept. 14, 1971 Assignee Danton A/S Nordborg, Denmark Priority Aug. 29,1967 Germany D 53 958 VllIb/Zlc GENERATOR FOR PRODUCING A WIDTH-MODULATED SQUARE VOLTAGE [0 Claims, 8 Drawing Figs.

Int. Cl H02 5/16 Field of Search 307/263,

[56] References Cited UNITED STATES PATENTS 3,402,353 9/1968 Hubbs307/265 3,440,566 4/ l 969 Swanson 307/263 Primary Examiner-Otis L.Rader Assistant Examiner-Thomas Langer Attorney- Wayne B. Easton l I2 I6I,

a-puAse suummou ZERO-LEVEL eeuzmnon uerwonx DETECTORS 9 ,29 5| 9,SOUAREWAVE PULSE- u R T R eeusuron b, ung:

PATENTEU SEPMISYI 3604895 SHEET 3 BF 3 2 H 5 3-PIHASEI suminou ZER-LEVELGENERATOR NETWORK V oETEcToRs 9 SQUAREWAVE PULSE- E GENERATOR I SHAPERINTEGRATOR 3-PHASE summon ZERO-LEVEL GENERATOR NETWORK DETECTORS g d n eT SQUAREWAVE PULSE- SLOPE GENERATOR SHAPER '"TEGRATOR nETEcToRCOMPARATOR B' 3-PHA5E suummou ZERO-LEVEL FIG. 7 GENERATOR NETWORKDETECTORS g 30 34 d 3 -e 55 SQUAREWAVE CONTROL SLOPE GENERATOR 6 UNIT'"TEGRATOR oETEcToR GENERATOR FOR PRODUCING A WIDTH-MODULATED SQUAREVOLTAGE This invention relates generally to wave generators and moreparticularly to a generator for producing a width-modulated squarevoltage, used in particular for controlling the speedofalternating-current motors, in which the width-modulated square voltageis obtained from a signal at a low frequency and one at a higherfrequency.

A wave generator is known in which a square wave voltage is obtainedfrom'the intersections of a constant frequency, constant amplitudetriangular voltage at high frequency and a variable frequency, variableamplitude sinusoidal voltage at low frequency. The frequency and theamplitude of the sinusoidal voltage are proportional in this case. Theresulting square voltage has constant amplitudes and may thus be takenfrom a direct current voltage source, for example, but correspondsnevertheless to the aforementioned sinusoidal voltage as regardsrepetition frequency and the given mean voltages I However, difficultiesare encountered with low frequencies when the amplitude of thesinusoidal voltage is small and many intersections occur" immediatelyadjacent zero level. Even small irregularities (temperature variations)in the circuit in which the low-frequency and higher-frequency signalsare compared will produce a useless or, at least, strongly falsiftedresult. With high frequencies the amplitude of the lowfrequency signalmay exceed that of the higher-frequency signal, so that intersections,and thus width modulation, may partly be lost altogether. Furthermore,it is not easy to simultaneously impart to the sinusoidal voltage avariable frequency and a variable amplitude either of which are meant toconform to a predetermined interdependence.

A principal object of the present invention is to provide a wavegenerator for producing a width-modulated square wave voltage generatedfrom a lower and a higher-frequency signal and which still operatesaccurately even when the frequency of the low-frequency signal assumesextreme values and which allows setting of amplitude and frequency ofthe desired square wave voltage with greater freedom than hithertopossible.

This is possible according to the invention, in that the lowfrequencysignal is of a constant amplitude but of a variable frequency and, inthe case of motor control, the low-frequency signal offsets thewidth-modulated square wave voltage to determine motor speed, and inthat the higher-frequency signal, also used in the wave generation ofthe invention, is of constant repetition frequency but of variableamplitude or slope and, in the case of motor control, offsets thewidthmodulated signal to determine the motor supply voltage. In thistype of generator the amplitude of the low-frequency signal ispreserved. Therefore, operating conditions under which thepoints ofintersection of the lower-frequency signal with the higher-frequencyfrequency signal would predominantly be within the zero-level regioncannot occur. Besides, the two variable parameters (frequency andamplitude) are each allocated to one of the two signals. As a result themodulation of the signals is greatly simplified and, above all, thevariation of frequency independently of amplitude, and vice versa,becomes possible. Consequently, both the frequency and the voltage levelof the width-modulated square wave can be much better controlled thanhitherto. the amplitude of the higher-frequency signal is supervisedseparately, it is possible to ensure that this amplitude is alwayshigher than the amplitude of the low-frequency signal.

The two signals are preferably applied separately to a summation networkin series with a zero-level detector ascertaining the sign of thesummation voltage and preferably operating with high amplification. Thezero-level detector can, therefore, deliver the desired square wavevoltage directly. Obviously, some other equivalent circuit element maybe provided, for. example, a Schmitt trigger. Another signal,corresponding to a. midpoint shift of'the connected load, may beadditionally applied to the summation network. Inthisway the zero-levelcan be accurately fixed, so that the intersections of the higherfrequency and the low-frequency signals accurately maintain the correctdistance from the zero or base line.

When applied to a control circuit for a three-phase motor, the generatorshould have three outputs for width-modulated square wave voltages whichoccupy the correct phase position. It is advisable to obtain the lowfrequency signal from the three-phase generator which has a summationnetwork connected to each of its three phases, while employing a singlephase, higher-frequency signal and feeding the latter signal to allthree summation networks in common.

In a preferred embodiment of the invention, both signals are produced byintegrating square waves the amplitudes of which are variable by meansof a direct-current voltage or direct current. This type of control ispossible because each signal is influenced only by one variable factor.Integration produces signals of varying slope, exploitable for amplitudecontrol as well as frequency control.

Thus, for example, the half-waves of the low-frequency signal may betrapeziums or trapezoidal waveforms with variable slopes. All thestraight sections of the trapeziums having a duration of 60 electricaldegrees. This trapezoidal-shaped wave is of particular advantage for athree-phase system because the sum of the instantaneous voltages isalways zero. The trapezium waveform approaches a sinusoidalcharacteristic sufficiently closely to avoid the third harmonic.Considering the fact that identical durations are specified for thethree straight sections of the trapezium waveform in conjunction withthe amplitude remaining constant, any variation of slope mustimmediately produce a variation of frequency. Moreover, such atrapezoidal characteristic can easily be produced with stationaryelectric components. Furthermore, the half-waves of the higher-frequencysignal may be basically triangular with variable slope but constantrepetition frequency. In view of its constant frequency, any variationof the slope of this signal must immediately vary its amplitude.

Since it is not necessary, in order to obtain intersections with thetriangular waves, to modulate these waves fully, it seems advisable tolimit the amplitude of the triangular basic wave to a valuesubstantially equivalent to the constant amplitude of the low-frequencysignal. Thus the voltage corresponding to the peak of the triangle neednot even appear in the system. In cases in which it is possible for theslope of the higher-frequency signal to become so fiat that theamplitude falls below the amplitude of the low-frequency signal, asignal with steeper edges can always be superimposed upon the triangularbasic wave in the amplitude region. The width of this superimposedsignal should preferably not fall below a predetermined value where theslope of the basic wave decreases below a predetermined minimum value.This ensures that two well-defined points of intersection with thelowfrequency signal will be obtained even in the least favorable case.

A particularly simple control arrangement is achieved when both signalsare variable in dependence on one and the same control direct-currentvoltage. In particular, the low-frequency signal may be renderedvariable in directly proportional manner, and the higherfrequency signalin inversely proportional manner and the higher-frequency signal ininversely proportional manner by means of one and the same directcurrentvoltage. In this way the repetition frequency of the square voltage aswell as its maximum mean value vary in proportion with one another, andthis results in a form of motor control whereby torque remains constanteven with varying speed.

In this case the direct current control voltage may be directly appliedto the control input of the generators for the low frequency andhigher-frequency frequency signals. Another embodiment provides for aslope detector to be arranged at the output of the generator producingthe higherfrequency signal. The slope-dependent direct-current voltageat the output of this detector is compared with the DC control voltagein an error detector and the resulting voltage is used to control theabove-mentioned generator. The feedback used in this arrangement servesthe formation of a reciprocal value of maximum accuracy. In otherarrangement the DC control voltage is only applied to the generatorproducing the higherfrequency signal; the latter is followed by a slopedetector and the direct-current voltage appearing at the output of thisdetector is used to control the generator which produces thelow-frequency signal.

A slope detector may be a device, for example, which, at a predeterminedlevel-preferably at the level of the constant amplitude of thelow-frequency signal-measures the distance between the two slopes of thehigh-frequency signal and delivers an DC signal proportional to thisdistance. This task may be performed by a circuit arrangement, forexample, in which the unaltered higher-frequency signal is applied tothe base of a first transistor while the same signal, chipped at thepredetermined level, is applied to the emitter of this transistor and inwhich the direct-current voltage component of this signal is obtainedfrom the resulting differential signal through a filter network. Ifnecessary after amplification, this component is finally applied to theregulating transistor of a directcurrent circuit which delivers thedesired control signal.

In a preferred embodiment the summation network adds the signalcurrents, for example, by applying the outputs outputs of the generatorsproducing the low frequency and the higherfrequency signals to a commonimpedance through a large ohmic resistance in each case of which atleast one of the resistors may be variable. This current additionenables the points of intersection to be obtained without any reactionon the rest of the circuit. Adaptation to different amplitudes in suchan arrangement is possible by linear networks.

Other features and advantages of the wave generator in accordance withthe present invention will be better understood as described in thefollowing specification and appended claims, in conjunction with thefollowing drawings in which:

FIG. 1, is a block diagram of a generator according to the invention;

FIG. 2, is a block diagram of an associate generator producing ahigher-frequency signal;

FIG. 3, is a block diagram of a unit in the associate generatorproducing low-frequency signals;

FIG. 4, is a waveform diagram illustrating the characteristics of thehigher-frequency signal, the low-frequency signals and the relevantsquare wave voltages in three-phase operation;

FIG. 5, is a block diagram of a first embodiment of a common controlarrangement or system for the generators producing the low frequency andthe higher-frequency signals;

FIG. 6, is a second embodiment of a control system of FIG.

FIG. 7, is a third embodiment of a control system of FIG. 5; and

FIG. 8, is a circuit diagram of a slope detector according to theinvention.

FIG. 1 illustrates a generator 1 producing a higher-frequency signal eof constant frequency the amplitude of which is however variable bymeans of a control voltage :8, and a three-phase generator 2 producinglow-frequency signals R,S, T of constant amplitude and of a frequencyvariable by means of the control voltage :8. The three-phase generator 2comprises three wave-generating units, 3, 4, 5 of identicalconstruction, series connected in cascade in a loop. The low frequencysignals R, S, T are applied to a summation network 6 in which each ofthese signals is added to the higher-frequency signal e. In order to dothis the signals in each case are applied to sets of resistors 7 and 8respectively and are them applied to common impedances 9. A resistor 10may also be provided in each case if necessary (only one of theseresistors is illustrated in the drawing the other are similarlyconnected to the respective junctions of the respective pair ofresistors 7, 8, and to a zero-level level voltage) and another signal,dependent on the zero-level shift in the load system, would then beadded. The sums of the signals are applied to zero-level detectors llcapable of high amplificatiomthe outputs of which make available widthmodulated square wave signals 3 9 9 As illustrated in FIG. 4 thezero-level detectors function to produce a square wave by switching fromoneoutput potential to another each time the difference between thesignal eand one of the signals R,S or T, attains a zero-level.

An embodiment for a generator 1 producing the higherfrequency signals oftriangular shape is illustrated in FIG. 2. An oscillator 12 generatespulses, for example at afrequency of 2,000 Hz. or cycles per second.These drive a flip-flop or bistable multivibrator 13 such that apositive and a negative direct-current voltage alternate at its outputlead 14, whereas the same voltage appears at another of its output leads15 with a 180 phase displacement. A double-ended limiter 16 keeps theamplitude of the first-mentioned voltage at a constant level, a constantfrequency and constant amplitude square voltage b is the result. In acontrol circuit or limiter circuit 17 also the output signals on thelead 15 are clipped at a predetermined amplitude proportional to thecontrol signal B. The clipped signals are applied to a nonlinearequalizing network 18 from which a constant frequency, variableamplitude square wave 0 is obtained. The signals b and c are applied toresistors 19 and 20 respectively to a summation network 21 in which theyare subtracted from one another, clue to their phase displacement. Theequalizing network 18 functions to produce its signal c at a levelwhereby differential signal d is approximately proportional to thereciprocal value of the control voltage B. Thus, to perform thisfunction, the network 18 must be nonlinear since the signal b is aconstant. The signal d likewise represents a constant frequency squarewave, is integrated by means of an amplifier 22 and an integratingcapacitor 23 included in a feedback circuit, so that a triangular signale results. The feedback circuit also includes a limiter 24 and anemitter followed 25. This ensures that the triangular signal e isclipped at a level predetermined by the limiter 24. Finally, asteep-edged signal, the amplitude of which is determined by the fullmodulation of the amplifier 22, is superimposed on the signal e thuslimited, in the region of the latters clipped amplitude. Thissteep-edged signal is superimposed by synchronously adding a suitablesquare wave to the signal e FIG. 3, is block diagram of the unit 5 ofthe three-phase generator 2 producing a low-frequency trapezoidalvoltage R. It will be understood that the other units 3, 4 of thethreephase generator 2 are similarly constructed. The output S of thepreceding unit 4 is applied to a high-amplification zero level detector46. Its output signal is a square wave. The amplitude of this wave isvaried in proportion with the control voltage B in a limiter circuit 47.The output signal a of the limiter 47 is thus a square wave of variableamplitude. This signal a is applied through a resistor 48 to anamplifier 49 shunted by an integrating capacitor 50. In parallel withthe capacitor is connected a limiter 51 consisting for example, of twoZener diodes, series-connected back to back and fixing the amplitude ofthe low-frequency signal. As a result of the loop or cascadearrangement, the three units 3-5 affect one another in such a way thatin each trapezoidal half-wave the duration of the straight sectionscorrespond to 60 electrical degrees. Therefore, if the slopes of thethree signals are varied simultaneously by means of the control voltageB, an instantaneous by means of the control voltage B, an instantaneousfrequency variation can be obtained.

In the second line of FIG. 4 the higher-frequency voltage e is shown bythe solid line and the three phases R, S, T of the lowfrequency voltageare shown by dotted lines. To facilitate the representation of thepoints of intersection, signal 2 has been displaced through 180 in thedrawing. It should also be noted that the frequency of signal e is muchhigher than indicated in the drawing. As an example it has a frequencyof 1,000 Hz. or cycles per second as against a frequency of 5 to Hz. orcycles per second in in the case of the low-frequency signals.

By following the points of intersection of the higherfrequency signaland the low-frequency signals the crossovers of the width-modulatedsquare voltages g g and 9 are obtained which are illustrated in thethird to fifth lines of FIG. 4.

By varying the slope of the higher-frequency signal esee the dash-dotcharacteristic in the second line of FIG. 4a square voltage is obtainedfor the phase R-see the dash-dot characteristics in the first line ofFIG. 4. It is immediately apparent that an increase in the slope leadsto a decrease of the mean values of the width-modulated square voltage.In the same way as a variation of the higher-frequency signal enablesthe mean value of the square wave to be varied, a frequency variation ofthe low frequency signal R, S, T will alter the mean value of the squarevoltage 3.

In FIG. 4, the rising slope, or leading edge section, the clippedamplitude section and the falling slope or trailing edge section of eachlow-frequency trapezoidal half-wave each corresponds to an electricalangle of 60. It also becomes apparent that the higher-frequency signal eexploits only a base portion 26 of the triangular voltage and is clippedat a level 27 which corresponds to the amplitude of the low-frequencysignal. Superimposed on this lower portion is a square wave signal 28which ensures that a point of intersection with the low-frequency signalis obtained at any rate. The square voltage 28 has a minimum width, sothat two clearly distinguishable points of intersection are obtainedeven under the most adverse conditions.

If it is desired that the torque of a motor should be independent offrequency, then the frequency and the supply voltage of the controlledmotor should be proportional to one another. In a generator such asproposed by the present invention this means that the control signal fimust vary the amplitude of the square wave responsible for the slope ofthe lowfrequency signal inversely proportional to the amplitude of thesquare responsible for the slope of the low-frequency signals. This canbe done by arranging for an appropriate transforma tion to take place inthe generator 2 for the low-frequency signals, see FIG. 3. Otherpossibilities will be discussed with reference to FIGS. 5 to 8, in whichreference numerals corresponding to those in the embodiments beforedescribed refer to similar elements.

In the embodiment of FIG. 5 the signal B is applied directly to thethree-phase generator 2 and also a pulse shaping device 29. The latterreceives from a square wave generator 30 which may be represented, forexample, by the units 12, 13 and 16 of FIG. 2, a square wave signal b ofconstant frequency and amplitude. The amplitude is multiplied with thereciprocal value 1/8 in the pulse-shaping network 29. The output signald of the pulse shaper 29 is integrated in the unit 31 consisting, forexample, of the units 22 to 25 of FIG. 2, and the resulting outputsignal 2 is applied to the summation network 6 and zerolevel detectorsII.

In the embodiment of FIG. 6 the circuit arrangement of FIG. 5 is stillfurther improved in that a comparison circuit or comparator 32 is linkedthrough a slope detector 33 later herein described, to the integratingcircuit 31 in a feedback loop. Since the is a function of the controlvoltage B, the signal transmitted by the comparison circuit 32 continuesto be corrected until the slope of the signal e corresponds to thecontrol voltage B. Since the slope and the control voltage B varyproportionally, the feedback signal can be directly compared with theinput control voltage B. The correct slope will have been attained whena zero signal appears at the output of the comparator 32.

In the embodiment shown in FIG. 7 the control voltage B is merelyapplied to a control unit 34 of the generator producing thelow-frequency signals. The corresponding control signal B for thethree-phase generator 2, on the other hand, is determined by measuringthe slope of the signal e by means of the slope detector 33. This hasthe advantage, first of all, that the frequency of the generator 2 isdirectly linked to the slope of the signal e fed to the summationcircuit 6 and also that a reciprocal value can automatically be producedby this circuit arrangement, i.e. B'=l/B.

A slope detector circuit suitable for the unit 33 is illustrated in FIG.8. The unchanged higher-frequency signal e is applied to the base of afirst transistor 35, the signal consisting of the base portion 26 andthe square wave 28. A signal e comprising only the base portion 26 isapplied to the emitter of this first transistor 35. As FIG. 2 shows,both signals are available in this circuit arrangement. The resultingdifferential signal is amplified in a second transistor 36. A filternetwork comprising the resistors 37, 38 the capacitors 39, 40 and avariable impedance 4 recovers the DC component which is applied to thebase of a transistor 42. This transistor is supplied from a voltagesource 43 and comprises in its emitter circuit two resistor 44 and 45across which the voltage +8 and B can be derived by means of which adouble-ended limiter arrangement in the three-phase generator 2 can becontrolled directly.

If, in FIG. 7, the square wave is integrates with the amplitude modifiedby B, then the signal e has a slope proportional to the value B. Thetransistor 35 conducts whenever the signal e is greater than the signale. The longer this transistor conducts, the flatter the slope of signale. It can be shown that the DC mean value of the differential signalproduced by the output transistor 35 is precisely inversely proportionalto the slope of signal e and can therefore be directly used, afteramplification and filtering, as the control signal Bto for thethree-phase generator 2.

Those skilled in the art will recognize that since the waves generatedare plus and minus going waves the control signal B has a plus and aminus polarity. The control signal B is a direct current voltage signalwhich may be developed, for example, in a motor control circuit, notshown, by a voltage divider across two direct current sources ofdifferent polarities. The generators can be connected in such a controlcircuit and the outputs 9 9 9,,, applied to control elements in themotor control circuit such as trigger means controlling switchesapplying the outputs to the three phases of the motor or a single phaseto maintain the motor speed constant. The variations in the directcontrol voltage will vary the amplitude of the signals generated by thegenerators as before described.

Furthermore, as to the superimposed signal above described it can begenerated by interrupting the feedback of the integrating circuit or bymaking it ineffective, by the limiter in the feedback circuit, when apredetermined signal voltage has been attained. The amplifier will thenimmediately assume a condition of full modulation, i.e. the base portionof the triangular signal attains the level of the predetermined signalvoltage and the square wave signal 28 is superimposed. This ensures thatthe predetermined level or amplitude of the triangular voltage or wavesis always exceeded by a safety margin.

While preferred embodiments of the invention have been shown anddescribed it will be understood that many modifications and changes canbe made within the true spirit and scope of the invention.

We claim:

1. Apparatus for producing a width-modulated square wave for controllingthe speed of alternating-current motors comprising, first generatormeans for generating a low frequency first signal defining analternating-current wave having trapezoidal shape in which each halfcycle is divided into three equal time regions, said low-frequencysignal having a constant amplitude and a variable frequency secondgenerator means generating a higher frequency second signal having aconstant repetition frequency and varying amplitude, and defining analternating current wave in which each half cycle defines a triangle;and means to receive the first and second signal and develop therefrom awidth-modulated square wave voltage.

2. The invention as set forth in claim I, in which said first generatormeans for generating a low-frequency signal comprises a three-phasegenerator including three identical units coupled together in a cascadearrangement, and each having an output conductor each said unitcomprising zero-level detector means having an output conductor andhaving an input conductor coupled to the output conductor of the justpreceding unit in said cascade arrangement, limiter means having a firstinput conductor coupled to the output conductor of its correspondingzero-detector means, said limiter means having a signal output conductorand a second input conductor for connection to a control signal forvarying the signal output amplitude therefrom, and integrating circuitmeans for integrating the output of said limiter means to generate saidwave defining a trapezoid at said integrating circuit means of each saidunit.

3. The invention as set forth in claim 1, in which said second generatormeans for generating a higher frequency comprises an oscillator having aoutput terminal, a flip-flop connected to said output terminal of saidoscillator and having two output terminals producing signals out ofphase with each other, two parallel limiters connected respectively tosaid two output terminals, each said limiter having an output conductor,an integrating circuit means connected to said two limiter outputconductors for summing and integrating the limiter outputs anddeveloping therefrom triangular waves corresponding to said secondsignal.

4. The invention as set forth in claim 3, in which said integratingcircuit means comprises means including an amplifier for superimposingon peaks of said second signal trianglar waves a steep-edged signal ofshorter duration than each triangular half cycle.

5. Apparatus according to claim 1, including means to apply a directcurrent voltage of plus and minus polarity to said first and said secondgenerator means for varying the frequency of the low-frequency signalproportionately to the value of said direct-current signal and varyingthe amplitude of the higherfrequency signal inversely proportional tothe value of said direct-current signal.

6. Apparatus according to claim 1, including means to apply adirect-current voltage of plus and minus polarity to said firstgenerator means to vary the frequency of the low frequency signalproportionately to the value of said direct-current signal.

7. Apparatus according to claim including 1, in which said second meanscomprises a square wave generator and an integrator connected in seriescombination with said square wave generator, said series combinationbeing connected in parallel with said first generator means, and saidintegrator having an output conductor, and in which said first generatormeans comprises a three-phase generator for generating three-phaseoutputs, a summation network having said three phase outputs connectedthereto, means applying the output conductor of the integrator to eachphase output received in said summation network, and a zero-leveldetector for each phase connected respectively in series with saidsummation networks.

8. Apparatus according to claim 1, in which said first generator meanscomprises a three-phase generator having three output conductors, and inwhich said second generator means comprises a square wave generator andan integration circuit connected in series combination, said combinationbeing connected in parallel with said three-phase generator, and asummation network connected to said three-phase generator for adding theoutput of said integrator to each output conductor of the three-phasegenerator, and means to apply a direct-current control signal of plusand minus polarity to said three-phase generator for controlling theamplitude of the outputs of the three-phase generator.

9. Apparatus according to claim 8, comprising a slope detector havingsaid integrator output conductor connected thereto for developing adirect-current control signal, to said first generator means to vary thefrequency of the low frequency signal proportionately to the value ofsaid directcurrent signal.

10. Apparatus according to claim 9, including a control circuitconnected between said integrator and said square wave generator inseries therewith said control circuit having means for receiving acontrol signal corresponding to said direct-current control signal.

1. Apparatus for producing a width-modulated square wave for controllingthe speed of alternating-current motors comprising, first generatormeans for generating a low frequency first signal defining analternating-current wave having trapezoidal shape in which each halfcycle is divided into three equal time regions, said low-frequencysignal having a constant amplitude and a variable frequency secondgenerator means generating a higher frequency second signal having aconstant repetition frequency and varying amplitude, and defining analternating current wave in which each half cycle defines a triangle;and means to receive the first and second signal and develop therefrom awidthmodulated square wave voltage.
 2. The invention as set forth inclaim 1, in which said first generator means for generating alow-frequency signal comprises a three-phase generator including threeidentical units coupled together in a cascade arrangement, and eachhaving an output conductor each said unit comprising zero-level detectormeans having an output conductor and having an input conductor coupledto the output conductor of the just preceding unit in said cascadearrangement, limiter means having a first input conductor coupled to theoutput conductor of its corresponding zero-detector means, said limitermeans having a signal output conductor and a second input conductor forconnection to a control signal for varying the signal output amplitudetherefrom, and integrating circuit means for integrating the output ofsaid limiter means to generate said wave defining a trapezoid at saidintegrating circuit means of each said unit.
 3. The invention as setforth in claim 1, in which said second generator means for generating ahigher frequency comprises an oscillator having a output terminal, aflip-flop connected to said output terminal of said oscillator andhaving two output terminals producing signals out of phase with eachother, two parallel limiters connected respectiveLy to said two outputterminals, each said limiter having an output conductor, an integratingcircuit means connected to said two limiter output conductors forsumming and integrating the limiter outputs and developing therefromtriangular waves corresponding to said second signal.
 4. The inventionas set forth in claim 3, in which said integrating circuit meanscomprises means including an amplifier for superimposing on peaks ofsaid second signal trianglar waves a steep-edged signal of shorterduration than each triangular half cycle.
 5. Apparatus according toclaim 1, including means to apply a direct current voltage of plus andminus polarity to said first and said second generator means for varyingthe frequency of the low-frequency signal proportionately to the valueof said direct-current signal and varying the amplitude of thehigher-frequency signal inversely proportional to the value of saiddirect-current signal.
 6. Apparatus according to claim 1, includingmeans to apply a direct-current voltage of plus and minus polarity tosaid first generator means to vary the frequency of the low frequencysignal proportionately to the value of said direct-current signal. 7.Apparatus according to claim including 1, in which said second meanscomprises a square wave generator and an integrator connected in seriescombination with said square wave generator, said series combinationbeing connected in parallel with said first generator means, and saidintegrator having an output conductor, and in which said first generatormeans comprises a three-phase generator for generating three-phaseoutputs, a summation network having said three phase outputs connectedthereto, means applying the output conductor of the integrator to eachphase output received in said summation network, and a zero-leveldetector for each phase connected respectively in series with saidsummation networks.
 8. Apparatus according to claim 1, in which saidfirst generator means comprises a three-phase generator having threeoutput conductors, and in which said second generator means comprises asquare wave generator and an integration circuit connected in seriescombination, said combination being connected in parallel with saidthree-phase generator, and a summation network connected to saidthree-phase generator for adding the output of said integrator to eachoutput conductor of the three-phase generator, and means to apply adirect-current control signal of plus and minus polarity to saidthree-phase generator for controlling the amplitude of the outputs ofthe three-phase generator.
 9. Apparatus according to claim 8, comprisinga slope detector having said integrator output conductor connectedthereto for developing a direct-current control signal, to said firstgenerator means to vary the frequency of the low frequency signalproportionately to the value of said direct-current signal. 10.Apparatus according to claim 9, including a control circuit connectedbetween said integrator and said square wave generator in seriestherewith said control circuit having means for receiving a controlsignal corresponding to said direct-current control signal.